In this work, we have investigated the design, fabrication and testing of ZnO-on-SOI fourth-order contour mode disk resonators for mass sensing applications. This study aims to unveil the possibility for real-time practical mass sensing applications by using high-Q ZnO-on-SOI contour-mode resonators while taking into account their unique modal characteristics. Through focused ion beam (FIB) direct-write metal deposition techniques, the effects of localized mass loading on the surface of three extensional mode devices have been investigated. Ten microfabricated 40 mm-radius disk resonators, which all have a 20 mm-thick silicon device layer and 1 mm-thick ZnO transducer layer but varied anchor widths and numbers, have exhibited resonant frequencies ranging from 84.9 MHz to 86.7 MHz with Q factors exceeding 6000 (in air) and 10,000 (in vacuum), respectively. It has been found that the added mass at the nodal locations leads to noticeable Q-factor degradation along with lower induced frequency drift, thereby resulting in reduced mass sensitivity. All three measured devices have shown a mass sensitivity of ~1.17 Hz·fg⁻¹ at the maximum displacement points with less than 33.3 ppm of deviation in term of fractional frequency change. This mass sensitivity is significantly higher than 0.334 Hz·fg⁻¹ at the nodal points. Moreover, the limit of detection (LOD) for this resonant mass sensor was determined to be 367 ag and 1290 ag (1 ag = 10⁻¹⁸ g) for loaded mass at the maximum and minimum displacement points, accordingly.

Motional resistance is one of the most important performance metrics for high qualitfy factor, low power, and CMOS-compatible capacitively transduced vibrating micromechanical resonators. The motional resistance is primarily set by the electrode-to-resonator air gap that can be formed by deep reactive ion etching (DRIE) process. Although the state-of-the-art DRIE technologies can achieve a narrow capacitive air gap down to 1μm or less, the effective gap tends to be larger than designed values due to the sidewall roughness known as scalloping. Systematic modifications of all key process parameters are presented in this paper for lowering the sidewall roughness to result in an up to 2x reduction of the effective capacitive transducer gap, which could lead to an up to 16x decrease of the effective motional resistance.

High motional resistance and incompatibility with post-CMOS fabrication due to thermal budget constraints are imperative issues associated with the back-end-of-line integration of lateral extensional vibrating micromechanical resonators. This paper presents piezoelectric ZnO-on-nickel resonators as a viable means for mitigating both of the issues. Lateral extensional mode resonators equipped with thin-film piezoelectric transducers can exhibit much lower motional impedances than their capacitive counterparts due to piezo-transducers’ higher electromechanical coupling coefficients. Meanwhile, the employment of electroplated nickel as the structural material allows the process temperature to be kept lower than 300 °C, which is low enough for the post-CMOS resonator fabrication. In this work, various geometrical rectangular and square plates resonators are investigated. Moreover, parallel combination of several resonators into a mechanically coupled array was explored as a systematic approach to lower motional resistance from ~1 kΩs to 0.562 kΩs. Higher order modes were investigated for achieving higher resonance frequencies up to 1.57 GHz. Local annealing by Joule heating was also exploited for quality factor improvement after device fabrication by ~2× enhancement and breaking the record of MEMS electroplated nickel resonators in lowering insertion loss to ~10 dB.

The authors wish to update the Acknowledgments section in their paper published in Actuators [1], doi:10.3390/act4020060, http://www.mdpi.com/2076-0825/4/2/60.